The present invention relates to a method of modifying the surface of hollow glass beads.
Hollow glass beads are often used as fillers for, for example, plastics materials and it will be appreciated that such beads should desirably conform with a number of criteria.
Firstly, the beads should be well formed for the purposes in view. For most practical purposes, this means that the beads should be closed and of similar size and density. Ensuring this according to current manufacturing techniques can give rise to considerable difficulties, so that in practice this means that a high proportion of any malformed beads present in a bulk supply of beads should be eliminated.
Secondly, the beads should have the correct bulk density for the purpose in view. It is usually important to have a low bulk density, and this is not always easy to achieve in the initial bead forming process.
Thirdly, we have found it desirable that the surfaces of the hollow glass beads should be polished. Such polishing removes surface defects from the glass so that the removed defects are no longer available as stress-raisers. Thus when comparing hollow glass beads of the same wall thickness, a polished bead will have a better mechanical resistance than an unpolished bead. The polishing of hollow glass beads is especially advantageous where the beads are to be used as filler in a resin. Stress distribution in a resin about a perfect sphere is uniform, symmetrical and predictable. Using polished beads as filler enables the mechanical properties of the filled resin to be more easily predicted and adjusted in view of the amount of filler used, and allows the production of a filled resin with homogeneous and isotropic mechanical properties. Of course when polishing a given hollow glass bead, the thickness of its walls will be reduced but there is nevertheless an improvement in the relative mechanical resistance of the bead. Since material is removed from the surfaces of glass beads by polishing, their bulk density will be reduced.